Engine size + RPM + MPH = Best MPG? - Fuelly Forums

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Old 09-14-2009, 01:42 AM   #1
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Engine size + RPM + MPH = Best MPG?

Has any study been done on calculating the most efficient speed based on RPMs for the best MPG?

Obviously, terrain & wind (head/tail from Mother nature, not wind resistance) affect MPG which the driver has little control over. Additionally, the biggest factor in getting the best MPG out of ANY GIVEN car is the driving style (no jack rabbit starts, anticipate stop lights, no hard breaking,....).

The question is "At what combination of RPM and speed with a given engine size (4, 6, or 8 cylinders) give a driver the best MPG?

Assumes: all other factors are the same (wind, hills, driving style).

It is my understanding (?) the optimal speed for the best MPG is around 45-50 mph. What I am looking for is a more definitive answer relating speed and RPMs to best MPG.

Example for clarification:

Obviously, a driver going 20 mph at 2000 rpm is not getting as good a mileage as one going 50 mph at 2000 rpm. (No brainer) This is the difference between driving in say 2nd gear and 4th/5th gear.

But a driver going 70 mph at 3000 rpm is covering more miles for the additional rpms (gas consumption) over the 50 mph/2000rpm driver. Were the additional 1000 rpms worth the additional miles covered in terms of MPG?

If a driver could go faster and not increase the RPMs significantly, the car could achieve a better MPG. Where is the break point in RPMs for a given engine size (4,6,8 cylinder)?

Any charts or graphs out there which show this information?
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Old 09-14-2009, 07:39 AM   #2
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You also need to take transmission/gearing into it too. With that said, the easiest way would be to get the BSFC map for a particular engine and go from there. Unfortunately, I don't really know where to get these, although if you have a Scangauge II you could probably map it out for your engine.
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Old 09-16-2009, 05:27 AM   #3
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As a general rule of thumb you're maximum mpg per rpm is at the lowest possible rpm that does not lug the engine in the highest possible gear. The reason for this is that drag increases with the square of speed. So to maximize your mileage you will see the best results going as slowly as you safely can in high gear at a constant speed. On a Freeway that will most likely occur somewhere around 55 MPH, on a highway you'll probably see better results at a somewhat slower speed as you won't be as big of a hazard going more slowly.

At some point you have to consider the value of your time and the danger that you put yourself and other drivers in when you go slower than the flow of traffic. In June and July of 2008 when gas was nearly $5.00 a gallon here in So. Cal. the flow of traffic on the LA freeway system slowed to around 50 MPH, that worked out wonderfully for me. My mileage went through the roof and I picked up 5-7 MPG by slowing down by 20 MPH on my commute. However as the months passed and the cost of gas fell, the freeway speeds increased back to the point where it is too dangerous for me to drive at less than 70 MPH under most circumstances. If I could get away with driving 30 MPH which would be just slightly above idle in high gear, I imagine that my mileage would increase by an additional 5-7 mpg but at a time cost and safety level that vastly exceeds the value in that increase.
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Old 09-16-2009, 07:49 AM   #4
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The thing you're also neglecting in your theorising lower rpm at the same speed = better economy is relative engine load. Also you're almost never on a perfectly flat, perfectly straight road, well in the UK anyway. Then you have to quantify when the ECU goes from closed loop cruising mode to open loop acceleration mode & that throws a spanner in the works too.

If you ignore the fiat figures for the latter 1.1 Seicentos & look at actual mpg on the road you find the LOWER geared Sporting at 60mph returns slightly better fuel economy than the non-sporting (same engine different 'box) models. Why? When laden or going up noticeable gradients the throttle is opened much further on the non-sportings as simply put the gearing is to high. More than that the most economical way to get up a 1:5 or steeper hill is to spin the engine to around 5000rpm! Driving at 60 in 5th & revving when going up a 20% or steeper incline was delivering high 50mpg to low 60mpg, but I'm talking about a free-revving 1108cc in a 800kg car that'd happily red-line with 50% throttle in 2nd.

If we look at my Panda, 1.4 16v turbo unit (home brew turbo conversion) in a 1000kg car, the best way to make progress under high load conditions, this also means accelerating, is to keep it at an rpm as low as possible with the turbo producing 0bar (relative) pressure. With loads of torque low in the rev-range for the chassis weight & a very nice piece of boost control you can use this torque to push you on with little effort.

On the Fiat 500 Abarth, the official 1.4 16v turbo, that doesn't have anywhere near as good a boost control system you're better off trying to keep the turbo off-boost until the engine can't cope then letting the turbo supply some boost but not a lot.

3 cars & 3 different ways to get the best economy out of them. In short is there are no hard & fast rules for every car, you have to work it out per-car. Personally I've seen that keeping an petrol engine much bellow 2000 rpm in high gears is a way to increase fuel usage latter in life & all fuel usage gains wiped out by premature big end failure (engine rebuild or new car time). Diesels rev lower & have different bottom end tolerances to cope with this, but I've not looked into oil burners.
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Old 09-17-2009, 06:53 PM   #5
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As well as cylinders, MPH, RPM etc you need to consider vehilcle mass and aerodynamic efficiency when trying to determine the ideal combination.

Also remember that engine load is more important than RPM, especially on small engines. For example, my olde Peugeot 107 1 litre petrol would give better consumption at 30mph when in 3rd gear when I could use a whisper of throttle - in 4th it used more fuel as I was having to give it a relatively large ammount of gas to keep it rolling at that velocity.

My relatively large (for a 4 pot petrol these days) 2.2 regularly gets North of 45mpg, yet weighs 1400kg dry and has a drag coefficient of CD 0.28. The number of variables make an absolutely staggering number of permutations, and there's probably more than one 'sweet' combination anyway.
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Old 09-25-2009, 03:11 PM   #6
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Thanks for your comments. It was informative and I appreciate it.
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Old 09-30-2009, 06:32 AM   #7
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Also, to consider, when purchasing that car, the smallest engine my not be the most fuel efficient. Often car manufactures, have three engine options, "Cheap, Good, and Faster." The "Cheap" engine, is likely to ware out sooner, and not necessarily get the beast fuel economy. It likely to be of an older design, and made of cast iron (very heavy). The "Good" option will be more optimized, likely will be fully made of Aluminum (lighter by a lot). And "Fast," is the HP option, not always the worst fuel economy...

If your commute requires you to drive over a mountain pass, the "Cheap" engine will require you to flore it, to dive at that roads minimum speed limit, it will likely get the same fuel economy as the "Fast" option. The Good option is likely give the most balanced between fuel economy and enough power, and reliability. I've owned only used cars with 100, 000 miles or more. The Bottom Line; the bottom line engine will be beat - heavy car, little engine, has been stressed (thrashed) all it life - these cars are the worst...
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Old 10-29-2009, 03:29 PM   #8
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I've always been interested in the efficiency/power of my engines. The MINI's OBC has a "constant econ" digital readout and I believe that traveling 68MPH in 5th @ 3000rpm is more fuel-efficient than 55MPH in 5th (or 4th). Partly due to having more torque/HP at higher speed permits the car to climb slight hills with less additional throttle required.

However, one day I drove along Skyline Drive's twisty roads in 3rd, revvying heavy, and econ still read 35mpg !? and avg the same at next fillup.

My suggestion for reducing consumption: MANTAIN SPEED!

I don't care if you're driving 55 or 70, just stay that speed.
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Old 10-30-2009, 09:28 AM   #9
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Yes, speeding up is what effects FE. Or worst having to stop and start from a stop. If all the stop signs were converted to traffic circles, I'd expect the see my in-town FE go from 23 to 25 MPG.

I think stop lights need to be redesigned to maximize through put. And, Laws set regarding yellow to red timing. As I don't like to see red when I leave the intersection, that when my car was interring the intersection, it was yellow?

I'd like to see traffic enforcement for slow driving, because this forces others to pass, and drive faster (wasting fuel), and making the drive, less safe...

These things will have a grater effect, than what gear, or speed you drive (assuming, you going at least the speed limit)...
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Old 11-08-2009, 10:00 AM   #10
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"I'd like to see traffic enforcement for slow driving, because this forces others to pass, and drive faster (wasting fuel), and making the drive, less safe...

These things will have a grater effect, than what gear, or speed you drive (assuming, you going at least the speed limit)..."

The majority of semi drivers would be ticketed if that were to happen.
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